The hypothesis that magnetic fields influence breast cancer risk is based on the existence of a plausible mechanism related to the effect of magnetic fields on melatonin along with some limited epidemiologic data. The study has two following specific goals. 1. To determine if residential exposure to magnetic fields, as assessed by wiring configuration coding and direct measurements is associated with the risk of breast cancer. 2. A secondary hypothesis is that particular combinations of the alternating current (AC) magnetic field and the direct current (DC) magnetic field, increase the risk of breast cancer. This hypothesis was prompted by observations of biologic effects at particular combinations of the AC and DC fields in several experimental systems. The study is a case-control study nested within a cohort study of risk factors for breast and other cancers among approximately 54,000 African-American and Latino women in Los Angeles County. A technician visits each case and control to make measurements and administer an interview on residential history and sources of exposure to magnetic fields and light at night, an effector of melatonin. We enrolled cases of breast cancer diagnosed over the 5 year study period and controls randomly sampled from the cohort. All cases of breast cancer diagnosed over the 5 year study period were being enrolled. We ended enrollment of subjects in November 1999. Since then we are completing wire mapping on subjects who either refused participation in the study or could not be reached before November 1999. Wire mapping provides an indirect measure of magnetic field exposure that can be done without entry into the subjects' property. We are currently doing the work necessary to complete and verify the data set so that analyses can begin. We have completed wire mapping on 727 cases and 697 controls. This represents more than 98% of eligible subjects that we attempted to contact. Interviews were completed on 426 cases and 420 controls. On 339 cases and 285 controls we have a complete set of magnetic field measurements, along with wire maps and questionnaires.